How could she be so sure? Dr. Shrier asked. Because, the patient replied, she had scoped out his body and "there's nothing irregular about him."

Dr. Shrier, a researcher on sexually transmitted infections, goes through this kind of conversation all the time. Patients tell her that they've never had blisters or lesions or sores, and so cannot possibly have genital herpes. The same for their sexual partners.

It falls to her to disabuse them of these notions, saying: "You can have lesions or not, you can have symptoms or not, you should basically be operating the same way, which is to assume that everyone has herpes." That means taking precautions, from limiting sexual contact to using condoms.

The study, led by Dr. Anna Wald of the University of Washington, found that people who'd had symptoms of herpes shed virus on about 20 percent of days, while people who test positive for herpes antibodies but have never had symptoms shed virus on only about 10 percent of days.

But here's the kicker: When they're shedding, people who've never had symptoms shed roughly the same amount of virus as people who've had symptoms. So it's clearer than ever that lack of symptoms is no guarantee against infection. And in fact, Dr. Wald said, "Asymptomatic shedding may be the central phenomenon of transmission."

In the old days, doctors would warn herpes patients to avoid sexual contact mainly when they had active lesions, believing that was the only time they were really contagious.

But evidence has long been growing that herpes can be transmitted even when no lesions are visible. The new study, by quantifying how much virus is shed even in the absence of symptoms, "is a real 'aha!' moment," said Fred Wyand, spokesman for the American Social Health Association. "It's really robust in terms of the number of subjects they enrolled and the length of time they were followed," he said.

The study also helps explain how genital herpes has become so wildly common, infecting nearly one-fifth of the American adult population, given that it's hard to imagine many people would want sex while they had the painful nether-regions equivalent of cold sores. Consider this stunning fact from the American Social Health Association:

In the United States, more people have genital herpes than all other sexually transmitted infections combined -– 50 million people in total.

There are more mind-boggling statistics.

Last week, NPR's Science Friday spoke with University of North Carolina professor of medicine Peter Leone about genital herpes. Here's a memorable piece of the transcript:

DR. LEONE: So if you look at some population-based data in the United States and look at unmarried adults, meaning folks between the ages of 45 and 50, for women, we know the prevalence rate for genital herpes due to HSV-2 is between 50 and 70 percent. So we're looking at close to the majority or the majority of folks in that age group who are single having genital herpes. So...

FLATOW: Let me go over that number again because you're saying that - it's hard to believe - you're saying that for women between the age of 40 and 50...

Dr. LEONE: Right.

FLATOW: ... unmarried women?

Dr. LEONE: Yes, unmarried women.

FLATOW: You're saying that between 50 and 75 percent of them have herpes type 2?

Dr. LEONE: That's exactly right. Yeah.

FLATOW: Wow.

Dr. LEONE: I know. It causes me to pause. I read the article about five times myself, saying, wow, and I do this for a living.

And that doesn't even tell the whole story. Dr. Shrier of Children's pointed out that "herpes is tricky:" genital herpes infections can also involve HSV-1, the virus known best as "cold sores" around the mouth. If someone with HSV-1 performs oral sex, the receiving partner may contract genital herpes, though it is HSV-1 rather than the typical 2. And HSV-1 is so widespread that pretty much everyone gets it by the time they're elderly, she said.

More reasons that herpes has spread so far and wide: It does not need intercourse to spread; mere skin-to-skin contact is enough. It can lie dormant for many years, suddenly cropping up in the midst of monogamous marriages.

And an obvious one: It has no cure, unlike chlamydia, gonorrhea and most other sexually transmitted diseases. Recent attempts to develop a vaccine fizzled, though more research is under way. Herpes patients can take antiviral drugs to shorten their outbreaks — and to become less infectious — but no existing pill can make it go away altogether.

Is testing the answer?So given that a huge swath of the population has genital herpes and that four-fifths of them don't even know it, is testing the way to go? If more people know they carry it, might they be more careful and become less likely to infect others?

Dr. Anna Wald argues that yes, more testing would help. Oftentimes, she said, when people start a relationship with a new partner and go for Sexually Transmitted Disease testing, it is not routine for a herpes antibody test to be included.

"There's this disconnect between the frequency of the disease and the impact it has on people's lives, and how we diagnose it," she said. "Because those couples have a relationship, and if one transmits it to the other, it could be devastating to both."

Herpes tests have improved a great deal in the last few years, she said, and "I believe in shared decision-making between the clinician and the patient, so I think if people are concerned about having herpes and want to be tested for it, it's reasonable to offer the test."

Dr. Shrier, too, said that she would arrange a test for a patient who wanted one, and that it can be helpful to identify "discordant couples" in which only one partner has herpes. But she does not support widespread screening.

"More than 50 million people in the United States have herpes Type 2, and the vast majority of them don't know they have it," she said. "If everybody went out and got tested, would it change anything? No. Just knowing you have the antibodies shouldn't necessarily drive much of anything because you should be practicing safer sex anyway. You just never know, and your partner never knows."